This invention relates to improvements in two-stage mast assemblies for lift trucks. More particularly the invention relates to a two-stage mast which provides a high degree of forward visibility for the lift truck operator while the truck is traveling with the load carriage substantially lowered and the mast substantially fully contracted.
One of the most common deficiencies of industrial lift trucks having telescopic masts at their forward ends is the problem of impaired operator forward visibility during travel of the lift truck with the load carriage in a substantially lowered position. In the environment where such trucks operate, a load is normally picked up at one location, lowered to a traveling position a few inches above the ground for stability, and then transported to another location where it is deposited. During such transporting of the load in the lowered position, the operator's forward line of vision is above the load so that the load and load carriage offer no obstruction. However in those types of masts where the hydraulic ram which operates the mast is centrally placed between the uprights, the ram usually obstructs the forward vision of the operator because the ram does not contract, even when the load carriage is lowered and the telescopic mast is fully contracted, to a low enough height that the operator's line of vision can pass over the top of the contracted ram. Thus the ram can cause a major blind spot in the operator's field of vision which can be especially serious in view of the congested environment of warehouse and loading dock operations.
Some mast constructions have solved this problem by utilizing a centrally located ram having a contracted height low enough to prevent obstruction of the operator's forward vision. To accomplish this objective, the ram must be designed such that the extensible end of the contracted ram is nearer to the longitudinal midpoint of the contracted mast than to the top thereof, and preferably at or below such midpoint. A major drawback of such constructions in the past is that they have required multiple, separately functioning sets of chains and sheaves within the mast structure in order to achieve the mechanical relationship necessary to extend the mast to its maximum extensibility and lift the load carriage through its maximum range of travel. Such multiple sets of chains and sheaves add substantial complexity and expense to the mast while being partially self-defeating of the visibility objective by taking up space in the mast structure which would otherwise be clear for forward viewing. For example, a two-stage mast of this type has previously been on the market which includes a first set of sheaves mounted at the extensible end of the ram over which is trained a first set of chains which interconnect the two mast sections so that, upon extension of the ram, the ram lifts one mast section relative to the other at twice the speed of extension of the ram. In addition a second set of chains, trained over a second set of sheaves at the top of the extensible mast section and interconnecting the base mast section with the load carriage, is required to move the load carriage with respect to the extensible mast section (at a rate of speed four times that of the ram). The first-mentioned set of chains must be unusually heavy in order to support the high chain loading due to the exceptionally great mechanical disadvantage of the chains relative to the load carriage.
Other types of two-stage masts have attempted to solve the visibility problem by eliminating the central location of the ram, and instead utilizing two transversely spaced rams such as shown for example in U.S. Pat. No. 4,030,568. Such structures present other types of difficulties however, particularly in that the transversely spaced rams occupy space which is otherwise needed for hydraulic hose reels and other mast accessory items, and add expense and weight as a result of the use of dual rams.